1. Field of the Invention
The present invention relates to voltage comparators, and more particularly, to a high speed voltage comparator having matched current sources using current difference amplifiers.
2. Description of the Related Art
A voltage comparator is a device used for comparing voltages. Voltage comparators have many applications, one example of which is in analog-to-digital (A/D) converters. In an A/D converter, each voltage comparator typically compares only two voltages. One voltage is the analog input voltage which is received by each comparator, and the other voltage is a fixed voltage which is different for each comparator. The outputs of the converters generate logic "highs" or logic "lows" depending upon which of their two input voltages is larger. In this way, the analog voltage is converted into digital units for digital computer processing.
Referring to FIGS. 1A and 1B there is illustrated a current difference amplifier 20 which is described in the paper entitled "High Resolution CMOS Current Comparators: Design and Applications to Current-Mode Function Generation", Analog Integrated Circuits and Signal Processing, 7, 149-165 (1995). The current difference amplifier 20 is basically a feedback loop formed by an inverter 22 and a non-inverting buffer 24. Transistors M1, M2 form the non-inverting buffer 24. When the input current i.sub.IN =0, transistors M1, M2 have the transfer function 26 shown in FIG. 1C. It should be noted that node V.sub.IN is actually the output of the non-inverting buffer 24 and that node V.sub.OUT is the input to the non-inverting buffer 24. Transistors M3, M4 form the inverter 22 which has the transfer function 28, also shown in FIG. 1C.
The interception point P of the transfer functions 26, 28 is the quiescent voltage level V.sub.Q at which the difference amplifier 20 operates when i.sub.IN =0. When enough current is sourced into the input node V.sub.IN, i.e., i.sub.IN &gt;0 (a "sourcing current"), transistor M2 turns on harder and the output voltage V.sub.OUT goes to zero, i.e., the operating point moves to point B on transfer function 28. When current is sinked from the input node V.sub.IN, i.e., i.sub.IN &lt;0 (a "sinking current"), transistor M1 turns on harder and the output voltage V.sub.OUT goes to V.sub.CC, i.e., the operating point moves to point A on transfer function 28. As is shown in FIG. 1C, the input voltage V.sub.IN changes very little and is biased near the inverter 22's switching point. The output voltage V.sub.OUT, however, swings from rail to rail depending on the input current i.sub.IN 's polarity and magnitude.
The current difference amplifier 20 was used in a current comparator which is described in the above-identified paper. However, there is a need for a high speed voltage comparator.